U.S. Pat. No. 5,325,166 to Hamilton et al. teaches assigning weighted values to sheets of different characteristics specifically the values of -7 for full size sheets, +13 for narrower sheets such as A5, and +17 for envelopes. These weighted values are accumulated as a count. When the count reaches a predetermined value, the time interval between subsequent sheets is increased. Subsequent feeding of full size sheets reduces the count to a second predetermined value at which normal throughput is resumed, although the second predetermined value is more than zero, so reduced throughput may be again resumed from feeding less sheets through from a cold condition.
The implementation of the foregoing patent was directed to nip fusers, which employ two rollers in nip relation through which the sheet passes for fusing. A belt fuser employs a thin belt wrapped over a ceramic or other low-thermal-capacity heater. A representative belt fuser is disclosed in U.S. Pat. No. 5,860,051 to Goto et al. The thin belt of a belt fuser is highly susceptible to damage from fusing sheets which extend only partly across the width of the belt, i.e., narrow media. The differential in heat across the belt is the source of overheat damage. Conversely, a belt fuser recovers more quickly from the differential in heat when no sheet is in the fuser, such as during the interpage time interval. This is particularly true where the heater is not powered during some of that time interval. The term "belt fuser characteristics" refers to a fuser having low thermal capacity, preferably one, which is routinely not powered during part of interpage time intervals.
To maintain good throughput (number of sheets per unit of time) while protecting the fuser, both width and length are determined and employed as described below.